Exam 3

Question 1

A eutectic material acheives most of it’s strength from?

Answer 1

Dispersion strengthening

Question 2

A seperate phase is always indicated by which of the following traits?

Answer 2

A change in composition

Question 3

In the Jominy test, a distance of 1 inch from the quenched end is how many Jominy distances?

Answer 3

16

Question 4

Eutectic alloys are useful due to their?

Answer 4

low melting temperature

Question 5

Intermetallic compounds are strong and hard due to?

Answer 5

Atomic (covalent) bonding

Question 6

“Hot Shortness” could be encountered during

Answer 6

solution treatment

and in the junior mechanical engineering class :0

Question 7

Describe the process of annealing an age-hardened material. Refer to the phase diagram, discuss time, temperature, microstructure, and mechanical properties:

Answer 7

Goal: soften the material
Result: Machinability, Ductility, improved impact-ability

Process: Heat up below the Solvus to disolve most of the theta- leaving only a little to serve as nucleation sites. As we slowly cool, the the hard theta will form at the nucleation sites instead of moving to the grain boundaries, which will increase ductility, as it is less of a barrier to dislocation motion.

Question 8

Describe the process of tempering 4340 steel?

Answer 8

1. Austenize: First step is to heat the part to the single phase austenite rergion. This gives a homogenous structure of FCC iron with interstitial carbons. Austinite is not stable at room temperature so we have to quench.
2. Quench: We want to get a structure of 100% body centred tetragonal martensite. We don’t need to allow time/temperature diffusion we merely need to cool fast enough to get to the martensite start line before we form any peralite or bainite. To find this cooling rate, we consult the Jominy chart and choose a distance before hardness drops off. Then we consult our specimen diamater, and see what medium we must cool in to achieve the correct cooling rate.
3. Temper: We want more ductility than pure martensite, but we don’t want to sacrifice the strength too much. Using a tempering temperature and properties chart, we can choose a temperature at which to temper the quenched metal to acheive the desired properties.

Question 9

Describe the process of annealing 4340 steel:

Answer 9

1. Austenitize:
   To anneal the steel, we will austinitize the same way as with the quench anf temper processes.
2. Slow Cooling:
   Then we will allow the part to cool at a much slower rate. As the part passes through the region between the A1 and A3 lines, farrite (alpha) will begin to nucleate at the austinite grain boundaries. (continous and ductile alpha matrix). Once the part reaches the eutectoid temperature, or the A1 temperature the remaining austenite is the eutectic composition. If we cool slowly, this will transform into coarse pearlite, giving good machinability and impact properties. A properties chart with annealing temperature and times can help us determine what the resulting mechanical properties will be.

Question 10

Nucleation occurs most easily at _______________?
Why?

Answer 10

Grain boundaries and existing planes (rock candy)
Because there is a lower energy level when a new phase forms where there is disorder in the lattice or on an existing surface.

Question 11

How do heat treatments control dispersion strengthening?

Answer 11

By controlling the development and distribution of the 2nd phase